There is no pattern to the ater comng out of the hot tap that is cold. I have tried it with the heating on and the heating off but sometimes it is hot and stays hot, other times it goes cold and never gets above luke warm

Try draining water from the water heater drain spigot, is it always hot? Now hold onto the hot pipe coming out of the heater while someone draws hot water. Does the temp of this pipe fluctuate over time?